This invention relates to a method and apparatus for forming a shoe shank on the bottom of a shoe insole to stiffen the shank region of the shoe. More particularly, the present invention is directed to further improvements in the methods and apparatus for applying, locating, retaining and curing a flexible strip of activatable thermosetting resinous material directly in situ on the shoe bottom so that the strip will conform to the contour of the shoe bottom and adhere thereto in stiffened, hardened form. Such strip materials are described in U.S. Pat. No. 4,081,917, issued Apr. 4, 1978; U.S. Pat. No. 4,122,573 issued Oct. 31, 1978 and others assigned to the assignee of this invention. The present invention relates to further improvements to the method and apparatus described in U.S. Pat. No. 4,161,048, issued July 17, 1979 to the assignee in this application, for applying the material to the shoe bottom.
The techniques and apparatus described in the foregoing patents have been receiving increased acceptance in the shoemaking industry in recent years. As described in further detail the above-mentioned patents, the use of the flexible, activatable strips to form a shank stiffener directly in situ on the shoe bottom solved numerous problems which had been presented in the prior art of shoe manufacture. Generally, as described in the above-mentioned patents, shank stiffeners typically have been inserted in shoes in the form of a stiff wood or steel preformed member. Because of the wide variety of styles and sizes of shoes, the typical prior practice required the manufacturer to maintain an inventory of a wide variety of different sizes and shapes of shanks. Many difficulties had been presented in the storage, proper selection and installation of such shanks. By utilizing the inventions described in the above-mentioned patents, a single flexible shank strip may be formed directly in place on the bottom of a shoe of any shape to conform precisely to the shape of the insolve bottom and then may be hardened in place in that shape by an external stimulus, such as radiant energy.
While in most instances, it is desirable to have a full longitudinal attachment of the shank strip to the insole bottom (sometimes referred to as a "dead fit") there may be some instances in which it is preferred that the heel portion of the shank strip be raised and spaced slightly from the heel region of the insole. This type of shank construction is sometimes referred to as a "spring fit" or a "spring shank" has been used in the prior art with metal shanks, particularly in the manufacture of some types of women's shoes. The purpose for using a spring shank during the manufacture of a shoe is to provide an automatic means to correct errors which may be made during the shoe manufacturing process. For example, if the shoe upper is not formed or cut precisely as intended, or if the upper is not pulled over properly, or if the heel height of the shoe is not just right, a spring shank may be incorporated to minimize the effects of these, and other, manufacturing errors.
In general, the object of the spring shank is to create a temporary spring force within the shoe assembly during the manufacture of the shoe, which spring force is released automatically when the shoe is finished and is removed from the last on which the shoe was formed.
During the manufacturing process, the shoe is retained firmly on the last and the tension or prestressing imposed by the spring shank has no effect on the shape of the shoe. However, when the shoe is finished and the last is removed, the spring then is free to return to its unstressed, natural configuration and will, of course, draw the shoe to the configuration of the shank. The natural relaxed configuration of the spring shank is in a more arched direction than the shoe was when it was on the last. Thus, when the last is removed, the shoe sole will be flexed in a more arched direction which will tend to tighten the top line of the shoe, smoothen out wrinkles in the upper and generally correct many of the manufacturing errors which may have occurred in the manufacturing process.
In general, the shoe manufacturing process, when using a spring shank, includes the location of a shank element on the bottom of the insole but in a manner in which the heel end of the shank will be raised slightly from the heel end of the insole. The shoe otherwise is manufactured in accordance with standard techniques, with the heel being attached toward the end of the manufacturing procedure. As the heel is applied, it is pressed against the raised heel end of the shank to decrease the extent of arch in the shank while simultaneously stressing the shank from is normal, more arched configuration to a stressed, less arched configuration. The manufacture of the shoe is completed with the shank in this prestressed configuration. Once the shoe has been finished and it is removed from the last, the shoe is free to flex under the influence of the prestressed shank. Thus, when the prestressed shank returns to its relaxed, more arched configuration, it will tend to cause the shoe to arch somewhat, thereby tightening the top line, the upper, and assuring a good tread.